import xml.etree.ElementTree as etree

tree = etree.parse('s100000022433.xml')
root = tree.getroot()
for adv in root.findall('adv'): # только подузлы 'adv'
    for Description in adv.findall('Description'): # только подузлы 'Description'
        phone = []
        plus = ''
        for f_100000022442 in Description.findall('f_100000022442'): # только подузлы 'f_100000022442'
            phone.append(f_100000022442.attrib.get('f_100000022443', '')+
                f_100000022442.attrib.get('f_100000022444', ''))
            plus = f_100000022442.attrib.get('f_100000022445', '')
            Description.remove(f_100000022442)
        Description.append(etree.fromstring('<f_100000022442 f_100000022444="{}" f_100000022445="{}"/>'.format(','.join(phone), plus)))

tree.write('строчки.xml',encoding='cp1251')
